How many hours in a Hebrew day in the Old Testament?

Answer 1

The Jewish day began at sunset. The Jewish Sabbath was celebrated from Sundown

Friday to after Sundown Saturday. Each day began at sundown. The length of the hours of daylight varied with the rotation of the earth, but there were always 24 hours in the day + night cycle.

Answer 2

The Hebrew word yom means "a time" and as such can be applied to any period of time although it is often used to mean "day". In Jewish religious practice a day begins at sundown and runs through to the next sundown, its length will therefore vary slightly from one day to the next through the year as the time of sundown shifts with the seasons, it is always aproximately 24 hours though.

It should be noted that in Biblical times the idea of a standard hour had not yet been developed. In the Old Testament period time of day was not marked off in increments at all. One would simply refer to how high the sun was in the sky during the day or how high the moon or certain stars were at night. By the New Testament period the idea of dividing the daylight period into hours had been invented but each day, from sunrise to sundown, had twelve hours to it. Thus the length of the hour would vary as the days got longer in the Summer and shorter in the Winter. Note that the Gospels say that Jesus was hung on the cross, "About the third hour." That means about three hours after sunrise or about 9:00 in the morning in Springtime. He died, "about the ninth hour." that would be nine hours after sunrise or about 3:00 in the afternoon.

This idea was invented by the Greeks and Romans though and whether the Jews would have used it would likely have been a matter of some debate between those who wished to assimilate into the larger Mediterranian culture and those who wished to preserve the distinct Jewish national and religious identity. The night though would still have been measured strictly in terms of the position of heavenly bodies.

The idea of measuring the day in terms of twenty four hours of standard length was not developed until the invention of accurate mechanical clocks in the fifteenth century.

Answer 3

The traditional stance is that each "yom" (day) of the six days of Creation was what we today call one day. Apologists for Evolution, on the other hand, hold that they were six longer periods of time. See also:

Is there evidence for Creation?

The use of divisions such as hours is ancient, and occurs in the Mishna (Berakhot ch.5) which dates back thousands of years ago. Clocks were not needed for this, since the sun's shadow by day, and the constellations by night, could be observed for measuring the passage of hours (Genesis 1:14, Rashi commentary, Berakhot 59b).